The present invention relates to an optical memory apparatus which records and plays back information optically. More particularly, it relates to an access device for positioning a light beam to a desired guide groove.
At present, an optical memory apparatus called "optical disc" has been proposed wherein a rotary disc, in which a predetermined substrate is vapor-coated with an information recording medium (e.g., a metal film), is irradiated with a laser beam which is focused into a spot of approximately 1 .mu.m in diameter and the irradiation power of which is modulated thereby to thermally provide recesses (called "pits") in the recording medium and to record information, and wherein at playback, a feeble laser beam is condensed and projected on the recording medium, and the information is read by utilizing variation in the quantity of reflected light from the pits. Such proposal has been made in `Electronics`, Nov. 23, 1978, p. 75, "Ten Billion Bits Fit onto Two Sides of 12-inch disc". A typical example of arrangement of the memory apparatus is shown in FIG. 1.
Here, a disc (partly broken away in the illustration of the figure) 3 having a diameter of approximately 30 cm is being rotated in the direction of an arrow about a rotary shaft 4 by a driving motor 5. An optical head 2, which is constructed of a laser source and an optical system, is carried on a swing arm actuator 1 having been used in a magnetic disc etc. and is driven in the radial direction of the disc 3.
The disc 3 has its surface covered with a transparent protective film 6 of glass or the like. In the figure, the protective film 6 is further partly broken away, to illustrate pits 12 to be described later.
Methods of recording and playing back information in such arrangement will be described with reference to FIG. 2 in which the illustrated part A of the disc 3 is shown on an enlarged scale.
On a substrate 11 of glass or plastic, guide grooves (called "tracks") 13 of a concave sectional structure having a certain degree of width and depth are formed by the use of an ultraviolet-setting resin 14 or the like. Further, a metal film 10 is evaporated on the resin 14, whereupon the protective film 6 is deposited. In case of recording, the focused spot of light from the optical head 2 is guided along the guide grooves 13, that is, the light spot tracks the guide grooves 13, and the metal film 10 is dissolved by the light spot so as to form the pits 12. In case of playback, a light spot is similarly projected along the guide grooves 13, and the quantity of the resulting reflected light is read.
Further, signals for controlling the light spot are detected from the quantity of the reflected light. The signals for controlling the light spot are, in the main, the two of a focal deviation detecting signal for detecting a focal deviation ascribable to the vertical oscillations of the disc, and a tracking deviation detecting signal for detecting the deviation between the center of the light spot and the center line of the guide groove. All such signals use the quantity of the reflected light from the metal film, namely, the area other than the pits.
Assuming the pitches of the guide grooves to be 1.6 .mu.m, one side of the disc having a diameter of 300 mm is provided with about 50,000 guide grooves, and data to be received per guide groove become about 4,000 bytes.
In each guide groove, a plurality of sectors for indicating the limits of the data are provided in the rotational direction of the disc in advance. In recording an external information at any desired position or playing back the recorded information, the access operation of looking for one guide groove out of the surface of the disc and finding out one sector on this guide groove is required. That is, there are required the so-called "seek" operation of moving the light spot to a selected guide groove where desired information is recorded or is to be recorded, and the so-called tracking of maintaining the light spot on the center line of the guide groove with the minimum deviation throughout the period of time during which the information is being read or being recorded.
As an apparatus requiring such access operation, there has heretofore been the magnetic disc. Since, however, the track pitches of the magnetic disc are about 150 .mu.m to 30 .mu.m and are one order to two orders greater than the pitches of the guide grooves of the optical disc, positioning based on the same access operation as in the magnetic disc cannot be applied to the optical disc. More specifically, when a magnetic head is brought to a desired track by an actuator (for example, a voice coil type linear motor), a steady state error (an offset from the target position) of about 5-10 .mu.m develops though it differs depending upon the construction and performance of the actuator. It is caused by friction etc. Moreover, in the transient state of the positional control, an overshoot can take place with respect to the target position, and it amounts to about 5 .mu.m. In this manner, with the access means employed in the magnetic disc, the stop precision is as very low as about 10 .mu.m. Since, as described before, the pitches of the guide grooves of the optical disc are about 1.6 .mu.m at the least for the time being, the positioning of the optical disc is difficult with the access method of the magnetic disc.
Besides, in cases where the actuator undergoes the maximum acceleration and the macimum deceleration, it is feared that the actuator itself will vibrate in the order of .mu.m. This leads to the problem that information from the disc cannot be read out during the seek control.
Furthermore, unlike the magnetic disc, the optical disc does not include a disc and a servo head for positioning. This leads to the problem that the exact position for the optical head cannot be detected.